Electrode method for the surface treatment of thermoplastic materials

ABSTRACT

In the corona discharge surface treatment of a thermoplastic material body wherein said body is positioned in an air gap between a pair of electrodes across which a high voltage is impressed, the improvement which comprises employing as one of said pair of electrodes a wire mesh comprising a plurality of relatively small diameter conductors spaced to provide a uniform high gradient field and the other of said pair of electrodes being a solid base electrode.

Unite tates Patent 1 1 Rosenthal Dec. 18, 1973 [54] ELECTRODE METHOD FORTHE SURFACE 3,632,299 1/1972 Thorsen 204/ 165 TREATMENT O THERMOPLASTIC1,796,110 3/1931 Lechler 204/312 X 3,318,790 5/1967 Carbajal et a1.204/165 X MATERIALS 3,443,980 5/1969 McBride 204/165 X [75] Inventor:Louis Aaron Rosenthal, Highland 3,491,009 l/l970 Ramaika..... 204/165 pk NJ, 3,600,122 8/1971 Coleman 204/165 x [73] Assignee: Union CarbideCorporation, New Primary Examiner p Edmundson York Attorney-Paul A. Roseand Aldo John Cozzi [22] Filed: Apr. 1, 1971 [57] ABSTRACT [21] Appl'130243 In the corona discharge surface treatment of a thermoplas'ticmaterial body wherein said body is posi- [52] US. Cl. 204/ 165, 250/531tioned in an air gap between a pair of electrodes [51] Int. Cl C08f47/22, HOlt 19/00 across which a high voltage is impressed, the improve-[58] Field of Search 204/165, 164, 212, ment which comprises employingas one of said pair of 204/223, 312, 323 electrodes a wire meshcomprising a plurality of relatively small diameter conductors spaced toprovide a [56] References Cited uniform high gradient field and theother of said pair UNITED STATES PATENTS of electrodes being a solidbase electrode. 3,440,418 4/1969 Piazza 204/312 3 Claims, 2 DrawingFigures bl/l/l/l/l /l/l/l/l/l I/I/I/I l/A ELECTRQDIE METHOD FOR THESURFACE TREATMENT OF THERMOPLASTIC MATERIALS The present inventionrelates to the corona discharge surface treatment of thermoplasticmaterials and, more particularly, to such treatment of surfaces ofthermoplastic material bodies to improve their adhesion to printinginks, paints, coatings, and bodies of other materials.

While prior systems for effecting such corona discharge surfacetreatment, employing bar, knife edge, flat and curved plate electrodes,produced generally adequate results, they provided problems inuniformity of distribution of corona discharge across the entire bodysurface to be treated with consequent nonuniformity in surfacetreatment.

It is, therefore, the prime object of the present invention to provide acorona discharge surface treatment system having improved uniformity oftreatment distribution.

Other aims and advantages of the invention will be apparent from thefollowing description and appended drawings.

In accordance with one aspect of the present inven tion, a process isprovided for the corona discharge surface treatment of a thermoplasticmaterial body, wherein said body is positioned in an air gap between apair of electrodes across which a high voltage is impressed, andcarrying out such treatment while employing as one of said .pair ofelectrodes a wire mesh comprising a plurality of relatively smalldiameter conductors spaced to provide a uniform high gradient field andas the other of said pair of electrodes, a solid base electrode.

As employed herein, the term wire mesh" is understood to mean a screenor wire cloth electrode comprised of a plurality of relatively smalldiameter conductors which are positioned in a uniformly-spaced manner soas to provide a uniform electrical field when the impressed voltage isapplied. An electrode, wherein the plurality of conductors are arrangedin two transversely crossing sets, is preferred. It is to be understood,however, that the wire mesh electrode may consist of a wire mail meshwhich may generally conform to the shape of the surface to be treated.Such a chain mail wire mesh electrode may be employed as long as the airgap spacing necessary for corona discharge film treating is provided.

Such wire mesh electrode system provides the following operationaladvantages:

1. It is possible to see the corona discharge and thereby assuringuniformity by permitting the adjustment of power output to achievevisual uniformity.

2. The wire mesh, because of the small radius wires producing highvoltage gradients, provides many nucleation sites for the coronadischarge streamers. There appears to be a tendency to uniformlydistribute the discharge and equalize the effects on variations in theair-gap and dielectric (thickness of the body being treated).

3. Awire mesh electrode permits the free flow of the air or other gassupporting the discharge.

4. A wire mesh electrode assists in proper electrode cooling and powerdissipation.

5. A deformable wire mesh electrode can be made to readily conform tothe surface contours of the body surface to be treated.

It has been found that some thermoplastic materials, particularly thosefilled with other materials such as fiberglass and electricallyconductive materials such as carbon black, metal powders or fibers andthe like, exhibit a dielectric weakness and cannot support theelectrical voltages normally encountered in a corona discharge (i.e.,8,000 to 30,000 volts). Tracking along the glass fibers and dielectricinhomogenieties are believed to be the cause for this dielectricweakness. Therefore, a buffer dielectric is preferably employed to aidin the voltage stress equalization. This dielectric also provides forcorona dispersion. For flat electrodes, glass is an ideal dielectric forthis purpose, but materials such as Hypalon (a commercial syntheticrubber having a good corona and ozone resistance), ceramics and poly:.

ester films can also be employed. The buffer dielectric is a practicalaccessory to mask dielectric weaknesses, but is not an essential aspectof this invention. Glass plates may be desirably positioned both betweenthe base electrodes and the material being treated and between the wiremesh and the air gap in which the corona discharge is maintained. [t is,however, to be noted that dielectric buffer plates do contributeelectrical losses to the treating system.

Buffering the electrodes with dielectric plates, although not essentialin the treatment of dielectrics, does provide the significant functionof dispersing the discharge, since a dielectric buffer insures that noparticular area can imbalance in a current grabbing" process. In thecase of poor dielectrics, such as carbonloaded materials and metallicfoils, buffering is mandatory. Double buffering, as well as double wiremesh electrodes, also contribute to the corona discharge uniformity anddistribution, particularly if both surfaces are to be uniformly treated.

In accordance with the apparatus aspect of the present invention,treatment apparatus is provided comprising in electric circuit relationwith a generator for corona discharge, a pair of spaced treatmentelectrodes, one of which is a wire mesh comprising a plurality ofrelatively small diameter conductors arranged to provide a uniform highgradient field and the other of which is a solid base electrode.

An electrode having a wire mesh of 14; inch is typically satisfactory.Too large a wire mesh opening results in a focusing action and producesa cross-hatched treatment pattern. Operability is not believed to becritical within any specific mesh size range.

The thermoplastic body to be corona discharge treated is backed at thegrounded side by a conducting base electrode which excludes air from theback side, which is not to be treated. In this connection, it is to benoted that it is not necessary to remove all corona from the back side.Incidental back side treatment is not harmful merely wasteful. Thisconducting surface may be an inflatable conducting rubber or a compliantconducting cloth or a conducting foam which deforms with the surfaceforced up against the underside by the inflatable member. The top sideof the other electrode is a wire mesh electrode shaped to conform to thesurface of the thermoplastic body to be treated but spaced therefrom byan air gap. This air gap can be from about 1/16 inches to :41. inches,and should be uniform across its length and width. In the case of flatbody treatment, the base plate and wire mesh electrodes may be rigid andflat.

In the drawing:

FIG. ll is a plan schematic view of corona discharge, top surface,treating apparatus embodying the inven tion; and

FIG. 2 is a sectional schematic view taken along the line 2-2 of FIG. I.7

Referring specifically to the embodiment of the drawing, generator 10 isprovided, having conductors l2 and 14 for energizing treating electrodes16 and 18, respectively. Treating electrode 16 is a wire mesh electrodehaving a plurality of uniformly-spaced, relatively small diameterconductors 20 arranged in the transversely-crossing wire mesh sets asshown in FIG. 1. Electrode 18 is a grounded solid base electrode.

Buffer dielectric means 22 and 24 may be provided between each of thetreating electrodes 16 and 18, respectively, and the air gap 26.

The thermoplastic material body to be treated 28 is positioned incontact with the solid base electrode 18 or its buffer dielectric means24, when employed. The material body to be treated may be eitherstatically positioned between the treating electrodes or passed as acontinuum (film or the like) through the treating zone between theelectrodes.

In examples of the practice of the invention, samples were treated byexposure to a -inch air gap, single glass buffered wire mesh electrodewith a 10 to-theinch mesh. Power input was 8.2 watts-square inch and(starting with exposures of seconds) it became apparent that one secondof exposure was sufficient for adequate treatment. Excellent subsequentadhesion to a foamed polyurethane material was obtained. These exampleswere carried out on fiberglass-filled polyethylene fiber material.

In another example of the treatment of a more difficult material(polypropylene glass reinforced), a typical power level was 270 wattsfor l0 minutes over a 4- inch by 10-inch area. This corresponds to 68wattminutes per square inch or 9.7 KW-minutes per square foot. As amatter of comparison, the energy density levels for typical polymerfilms are l to 2 watt minutes per square foot. This represents a widerange of treating efficacy which is apparently the result of the surfacechemistry. It was not possible to treat this material with solidelectrodes under any circumstances.

What is claimed is:

1. In a process for the corona discharge surface treatment of athermoplastic material body wherein said body is positioned in an airgap between a pair of electrodes across which a high voltage isimpressed, the improvement which comprises employing as one of said pairof electrodes a wire mesh comprising a plurality of relatively smalldiameter conductors spaced to provide a uniform high gradient field andgenerally conforming in shape to the shape of the surface of said bodyto be treated, and the other of said pair of electrodes being a solidbase electrode, while maintaining a buffer dielectric between the wiremesh electrode and said thermoplastic material body.

2. A process as claimed in claim 1, wherein said ther moplastic body tobe treated is maintained during treatment in contact with said baseelectrode and spaced from said wire mesh electrode to provide an air gaptherebetween.

3. A process as claimed in claim 1, wyerein treatment is carried outwhile maintaining a buffer dielectric positioned between each of saidelectrodes and said thermoplastic material body.

2. A process as claimed in claim 1, wherein said thermoplastic body tobe treated is maintained during treatment in contact with said baseelectrode and spaced from said wire mesh electrode to provide an air gaptherebetween.
 3. A process as claimed in claim 1, wyerein treatment iscarried out while maintaining a buffer dielectric positioned betweeneach of said electrodes and said thermoplastic material body.